The influence of electrical interactions on the composition of Langmuir films is theoretically studied. The overlapping electric double layers in the region close to the three-phase contact line are described on the basis of the Poisson-Boltzmann equation, taking into account the dissociation of fatty acids and the counterion binding to the surface film. It is concluded that the composition of the monolayer changes insignificantly during the deposition at the considered equilibrium constants and ionic concentrations in the solution. The results of the theory are used to discuss instability mechanisms during the Langmuir wetting process.